A really big earthquake could cut California off of the mainland. It’s a patchwork of smaller tectonic plates crunched up, and the lot of them getting crunched up against the mainland plate. Or maybe in this universe it hasn’t yet gotten crucnhed up against the mainland plate far enough for it to have emerged from the sea.

The Himalayas have and currently still are being formed the same way as most continental border mountains, and the Rocky Mountains and Andes are all part of one long mountain chain on the west end of the large continental plates. California’s basically an extra piece that’s getting pushed against the Rockies by the same function, hence it getting some extra mountains of its own.

I’m more curious about what looks like a VERY large lake east of Ultra Nevada.